The present invention relates to the structures of module units having electronic parts and waterproof connectors mounted on circuit boards and encapsulated therein by resin molding. In particular, the invention relates to a structure suitable for engine control units and other electronic control units installed in automobiles, ships, agricultural machines, engineering machines, electric generators, etc.
Recently, the thermal environments of engine control modules (hereinafter, denoted as “engine control units”) used in automobiles, ships, agricultural machines, engineering machines and electric generators are getting harsher and harsher.
That is, the situation is: (1) while they were conventionally installed in cabins, installation in engine compartments and on-engine mount are becoming popular, exposing engine control units to higher temperatures; (2) the amounts of heat generated by engine control units are increasing due to the increasing load current to be controlled; and (3) due to their downsizing, the amounts of heat generated by engine control units are increasing per unit volume as well.
Conventionally, typical engine control units employ a non-watertight structure comprising a printed circuit board having electronic parts mounted thereon and a case to cover them. For installation in engine compartments, inexpensive engine control units have appeared with cases which are structurally improved in heat radiation and resistance.
However, on-engine mount units are used in a still harsher thermal environment. They are required to endure 120° C. or higher temperatures. Engine control units for installation in engine compartments cannot operate at temperatures higher than 100° C. and therefore can not cope with such an environment.
Although there are engine control units which employ a high heat radiation structure comprising a ceramic board and bare chips, this structure is expensive as compared to the conventional structure.
As a conventional technology to solve these problems, Japanese Patent Laid-Open No. 2004-111435 discloses a resin-molded high heat radiation and high reliability engine control unit capable of enduring higher temperatures and higher thermal dissipation.
In addition, Japanese Patent Laid-Open No. 1996-70066 discloses a structure in which a cavity is provided on only one side of a circuit board in a semiconductor device.
The conventional technology of Japanese Patent Laid-Open No. 2004-111435, which encapsulates all electronic parts by resin molding, has various problems.
For example, the internal electrolytic solution of an electrolytic capacitor used in an engine control unit usually causes a small amount of leakage to the outside due to gasification. Therefore, if the electrolytic capacitor is sealed with resin, the gasified electrolyte solution accumulates near an electrode of the electrolytic capacitor and may result in causing a short circuit. Although an electrolytic capacitor is usually provided with an explosion-protection valve to release the gas pressure, the gas pressure may rise since this function is disabled if the electrolytic capacitor is sealed with resin. Therefore, using this conventional technology with electrolytic capacitors is not practical.
Although ceramic capacitors, tantalum capacitors and like can be used as substitutes, they are expensive and have a disadvantage in terms of cost. In addition, while common rail systems are becoming popular for low-emission diesel engines in Europe, their engine control units must contain capacitors having large capacitance values which neither the ceramic capacitor nor the tantalum capacitor can have. Therefore, this requirement can not be met if the control unit is encapsulated simply by resin molding.
In addition, large coils used in engine control units are likely to change in properties if they are sealed with resin. Therefore, it is not preferable to seal coils with resin in resin-molded engine control units.
Further, each electronic control unit is recently required to be provided with a self-diagnosis function to detect abnormality in the system. If the system malfunctions, the system must record details of the abnormality in an internal storage device to allow later investigation. However, if the electronic unit is encapsulated by resin molding, it is not possible to probe or visually inspect the internal circuit and electronic elements after the occurrence of a trouble in the electronic circuit. This makes it difficult to determine the causes of abnormalities of the system.
In the case of Japanese Patent Laid-Open No. 1996-70066 which discloses a structure where a cavity is provided on only one side of the circuit board, it is not possible to contain large electronic parts such as pin insertion type ones although this is effective to semiconductor and other small parts.